We expected that the attractive force of gravity would slow down the rate at which the universe is expanding. But observations of very distant exploding stars (supernovae) show that the expansion rate is actually speeding up. Filippenko, a member of teams that revealed the accelerating expansion of the universe, will discuss some of the implications of this phenomenon as it relates to current research. The universe seems to be dominated by a repulsive “dark energy” – an idea Albert Einstein had suggested in 1917 but renounced in 1929 as his “biggest blunder.” It stretches the fabric of space itself faster and faster with time, creating a “runaway universe.” The physical origin and nature of dark energy, which makes up about three quarters of the contents of the universe, is probably the most important unsolved problem in all of physics.